As a conventional driving system having a voltage conversion apparatus, there is proposed a system comprising, for example, a battery which serves as a power source of the driving system, a DC/DC converter for performing DC/DC conversion to an input voltage from the battery, a load comprising an inverter circuit for converting an output from the DC/DC converter into multi-phase AC electrical power and a motor for rotation driving while receiving the multi-phase AC electrical power from the inverter circuit, and a capacitor arranged between the DC/DC converter and the load and connected to the positive and negative bus bars (lines) of the inverter circuit (for example, Japanese Patent Laid-open Publication No. 2004-112904).
In this system, an input voltage from the battery is subjected to DC/DC conversion by the DC/DC converter, and thereafter stored in the capacitor, so that a load is driven using the power-storing capacitor which is then considered as a DC power source. Also, the voltage conversion ratio of the DC/DC converter can be controlled by adjusting the duty ratio of a switching control signal to be output to a switching element within the DC/DC converter.
Also, International Publication No. 03/61104 discloses a control apparatus of a voltage conversion apparatus.
For the reasons described below, such a system requires control to limit the voltage conversion ratio of the DC/DC converter by limiting the duty ratio of a switching control signal. Generally, a typical battery is designed capable of supplying electrical power of an amount corresponding to load requiring power. Depending on the condition of a battery, however, a battery may not be able to output electrical power of an amount corresponding to the load requiring power, for example, when the internal resistance of a battery increases due to drop of temperature.
In this case, should a DC/DC converter be simply controlled to be driven such that electrical power of an amount corresponding to the load requiring power is fed to the load, an increased amount of electrical power may be consumed due to the inner resistance of the battery and, consequently, only a reduced amount of electrical power can be supplied to the load. In order to suppress drop in the electrical power supplied to the load, the duty ratio of a switching control signal requires limitation.
According to the international publication No. 03/61104, the control apparatus of the voltage conversion apparatus sets the duty ratio which leads to the maximum battery power as a lower limit of the optimum duty range, based on the electromotive voltage (which is set as a constant) of a battery and an output voltage of a DC/DC converter, and the duty ratio of a switching control signal to be supplied to the switching element is limited so as to remain within the optimum duty range. That is, the related control apparatus of the voltage conversion apparatus limits the duty ratio to thereby suppress drop of electrical power supplied to the load.
In this related art, however, as the electromotive voltage of a battery is set as a constant, when an electromotive voltage varies due to variation in the state of charge (SOC) of the battery, the lower limit of the optimum duty range may also undesirably vary due to variation in the state of charge (SOC) of the battery. This makes it difficult to perform optimum control to limit the voltage conversion ratio of the DC/DC converter.
The present invention can advantageously provide a control apparatus and a control method for a voltage conversion apparatus capable of performing more appropriate control to limit the voltage conversion ratio of the voltage conversion apparatus.